Conventional silver halide colour photographic elements contain a number of silver halide emulsion layers. These layers are spectrally sensitized to particular colours of light and have associated therewith image dye-forming coupler compounds, hereinafter referred to as `couplers`, capable of forming image dyes upon contact with oxidised developer. In order to obtain an image in a desirable processing time, it is necessary that the coupler have sufficient activity to form the image in the allowed processing time. This requires a certain minimum range of reactivity. When the image is ultimately comprised of more than one dye, it is further necessary that the reactivity of the couplers of different colours be balanced so as to enable a proper neutral colour when needed.
When dye images are formed in silver halide photographic materials from the combination of oxidised developer and incorporated image dye-forming couplers, certain stringent demands are made of the couplers. For instance, the couplers must yield dyes which absorb in the correct region of the visible spectrum and which are resistant to fading by light, heat and humidity. The couplers must be active, have a low propensity to form fog, be easily dispersible and must themselves be resistant to the deleterious actions of light, heat and humidity. It is also important that the photographic performance of the couplers is resistant to changes in processing conditions, such as changes in the pH of the developing solution.
In the design of a coupler, it is the aim of the photographic chemist to incorporate various groups into the coupler structure to achieve, or partially achieve, the various desirable properties outlined above. However, it is well known in the art of coupler chemistry that when a functionality is incorporated into a molecule to achieve one of the aforementioned desirable properties (such as high activity), quite often one of the other desirable properties (such as high dye light stability or dye hue) is adversely affected. The structure of a coupler is therefore always a compromise. Furthermore, couplers which contain many functionalities suffer from the disadvantage that they are difficult and therefore expensive to prepare.
For example, hydrophilic substituents on couplers have been used effectively to improve the activity of the coupler through lowering the apparent dispersion pKa (known as pH1/2) of the coupler and through increasing the hydrophilicity of the coupler/coupler solvent particles.
However, such substituents have adverse effects on other aspects of the coupler such as ease of synthesis, cost and degradation in light stability and shift in dye hue of the resulting dye. In addition, incorporation of solubilizing/activating groups into the coupler can lead to an increased tendency for the image dye and/or the coupler to `wash out` from the element during the process.
The use of addenda to improve specific properties of the image coupler or the resultant photographic dye formed on development is well established. These addenda may be added in the form of a co-dispersion with the image coupler or added separately to the photographic layer either as a separate dispersion or in solution. For example it is common practice to enhance stability of photographic dyes to light by the use of addenda such as poly-alkoxy benzenes or substituted phenols or bis-phenols. In U.S. Pat. No. 4,727,015 the incorporation of organosilanes in a photographic layer containing a dye-forming coupler is shown to improve the density and contrast of dye images produced after exposure and processing. U.S. Pat. No. 4,840,877 discloses the use of ballasted carboxylic acids with magenta dye-forming couplers to improve coupler/silver interactions which may lead to speed losses and to their use to improve the efficiency of dye formation. U.S. Pat. No.5,382,500 discloses sulfonamides in green sensitive layers giving materials with improved keeping and process pH sensitivity. JP 07209839 describes sulfonamides as developing accelerators in heat developable systems in combination with dye releasing reductants. U.S. Pat. No. 4,973,535 and EP 0 510 576 disclose respectively the use of sulfonamide solvents to alter the hue of couplers and sulfoxides to lessen continued coupling in magenta couplers. U.S. Pat. No. 5,120,636 claims a combination of magenta coupler, sulfonamide and bis-phenol for improving light fastness and U.S. Pat. No. 4,898,811 describes sulfonylphenols as oil formers.
Problem to be Solved by the Invention
The problem is to provide a means for improving photographic performance of silver halide elements, and in particular improving dye yield and robustness to process pH variation, resulting in higher and more consistent density.